1. Field of Invention
This invention relates generally to mass flowmeters, and more particularly to a Coriolis-type meter of the doubleloop type in which the loops are so anchored as to define a tuning fork in which the loops on either side of the anchored center which is the junction of the loops are free to vibrate in phase opposition and t torsionally oscillate.
2. Status of Art
A mass flow rate meter is an instrument for measuring the mass of a fluid flowing through a conduit per unit time. Most meters for this purpose measure a quantity from which the mass can be inferred, rather than measuring mass directly. Thus, one can measure the mass flow rate with a volumetric flowmeter by also taking into account pressure, temperature and other parameters to compute the mass.
A Coriolis-type mass flowmeter, which is also called a Coriolis/Gyroscopic meter, provides an output directly proportional to mass flow, thereby obviating the need to measure pressure, temperature, density and other parameters. In this type of meter, there are no obstacles in the path of the flowing fluid, and the accuracy of the instrument is unaffected by erosion, corrosion or scale build-up in the flow sensor.
The theory underlying a Coriolis-type mass flowmeter and the advantages gained thereby are spelled out in the article by K.O. Plache, "Coriolis/Gyroscopic Flow Meter" in the March 1979 issue of Mechanical Engineering, pages 36 to 39.
A Coriolis force is generally associated with a continuously rotating system. Thus, the earth's rotation causes winds from a high pressure region to spiral outwardly in a clockwise direction in the northern hemisphere, and in the counterclockwise direction in the southern hemisphere. And a person moving on a merry-go-round will experience a lateral force and must lean sideways in order to move forward when walking outward along a radius.
A Coriolis force and precession in a gyroscope arise from the same principle. In a gyroscope, when a torque is applied at right angles to the axis of rotor spin, this will produce a precessional rotation at right angles to the spin axis and to the applied torque axis. A Coriolis force involves the radial movement of mass from one point on a rotating body to a second point, as a result of which the peripheral velocity of the mass is caused to accelerate. This acceleration of the mass generates a force in the plane of rotation which is normal to the instantaneous radial movement.
In one known form of Coriolis-type mass flowmeter, the fluid to be metered flows through a C-shaped pipe which, in association with a leaf spring, act as the opposing tines of a tuning fork. This fork is electromagnetically actuated, thereby subjecting each moving particle within the pipe to a Coriolis-type acceleration. The resultant forces angularly deflect or twist the C-shaped pipe to a degree inversely proportional to the stiffness of the pipe and directly proportional to the mass flow rate within the pipe.
The twist of the pipe is electro-optically sensed twice during each cycle of tuning fork oscillation which takes place at the natural resonance frequency of the structure. The output of the optical detector is a pulse whose width is modulated as a function of the mass flow rate. This pulse width is digitized and displayed to provide a numerical indication of mass flow rate.
In the Roth U.S. Pat. No. 3,132,512, a Coriolis-type mass flowmeter is disclosed in which a flow loop vibrating at its resonance frequency is caused to oscillate about a torque axis which varies with fluid flow in the loop. This torsional oscillation is sensed by moving coil transducers.
The Cox et al. U.S. Pat. Nos. 4,127,828 and 4,192,184 show a Coriolis-type meter having two U-shaped flow loops arranged to vibrate like the tines of a tuning fork, torsional oscillation of these loops in accordance with the mass of the fluid passing therethrough being sensed by light detectors. In the Smith U.S. Pat. No. 4,222,338, electromagnetic sensors provide a linear analog signal representing the oscillatory motion of a U-shaped pipe. Electromagnetic sensors are also used in Smith et al., U.S. Pat. No. 4,491,025, in which the fluid whose mass is to be measured flows serially through two parallel U-shaped pipes which together operate as the tines of a tuning fork.
Also of background interest are the following references:
Sipin--U.S. Pat. No. 3,485,098
Smith--U.S. Pat. No. 4,187,721
Smith et at.--U.S. Pat. No. 4,252,028
Dahlin--International patent application No. 85/05677
Because a double-loop Coriolis type meter functions as a tuning fork, much less power is required to oscillate the two loops at their natural frequency than would be required to oscillate one loop alone. When the two loops vibrate as a tuning fork with respect to an anchored center at the junction of the two loops, they will alternately come close together to a minimum spacing and then separate to a maximum spacing; hence the angular velocity vector for one loop will always be opposite to the angular velocity vector for the other loop. And because the flow through the two loops is the same, the loops will be subjected to opposing torques by reason of the opposite angular velocity vectors. As a consequence, the two loops are caused alternately to twist toward and away from each other.
A double-loop tuning fork configuration provides a more stable operation than a single loop mass flowmeter; for as the mass of one loop varies due to increased fluid density, so will the mass of the other loop. This results in a dynamically balanced pair of loops and a substantially decreased sensitivity to external vibratory forces.
However, because the loops of the tuning fork are anchored at their center which is the junction of the two loops as well as the inlet and outlet ends, such anchoring strongly inhibits deflection of the loops. As a result, velocity sensors of the type used in the prior art are not sufficiently sensitive to provide an adequate signal for mass flow measurement.